William Hague yesterday intensified his attack on the government for increasing Britain's overall tax burden when he claimed that business would be forced to pay billions of pounds more than the chancellor forecast in his Budget.

Taxes on business would rise by £30bn over the lifetime of the current parliament, blowing a hole in Tony Blair's claim that such taxes were falling, the Tory leader told the Commons.

To underline his point, Mr Hague seized on claims by the accountancy firm PricewaterhouseCoopers which estimated that two measures in the Budget would cost multinational companies several billion pounds a year, rather than the £300m estimated by the Treasury.

In an overnight Budget report, PwC said that the Treasury had committed a "simple error" in its calculations over the impact on business of the controlled foreign companies and double tax relief. Double tax relief has been used for the past 30 years by American and Japanese companies legally to avoid tax on income from various European operations.

The two measures are designed to crack down on multinational companies hiding their overseas profits to avoid paying British tax. One well known multi national told PwC yesterday that the changes in the Budget would cost it at least £1bn. "There are far more attractive regimes in countries such as Denmark and Spain ... the chancellor will drive out business from the UK," said PwC.

Mr Hague, who is determined to place tax at the heart of the Tories' election campaign, asked the prime minister whether he was "completely confident" about the chancellor's figures in the light of PwC's claims. Mr Blair said he believed the accountancy firm had misunderstood the Budget. "We have asked them to discuss them with us," he told MPs.

The prime minister's remarks were dismissed by Peter Wyman, a partner at PwC who raised doubts about the taxes. "This is not a misunderstanding, we understand Treasury press releases perfectly clearly," he said. "We have extreme concern over the way in which [these] changes will affect international companies."

Nicholas Dee, chairman of the Confederation of British Industry's tax committee, said the CBI would be "protesting vigorously" to the treasury about the proposed changes.

Apart from hitting companies' balance sheets by more than the Treasury estimates, the tax would deter British companies from investing abroad and act as a barrier to cross-border mergers, he said.